Suspension systems for motor vehicles

ABSTRACT

A suspension system for a motor vehicle includes a shock absorber having a device for allowing a change in a damping-force characteristic, a first sensor for sensing a vertical behavior of the motor vehicle, a second sensor for sensing a roll angle of the motor vehicle during steering operation, a third sensor for sensing a roll rate of the motor vehicle during steering operation, and a control unit. The control unit includes a fundamental control part for carrying out an ordinary control of the damping-force characteristic of the shock absorber in accordance with the vertical behavior as sensed, a first roll restraining control part for carrying out a first roll restraining control in place of the ordinary control when the roll angle as sensed is equal to or greater than a predetermined threshold value, and a second roll restraining control part for carrying out a second roll restraining control in place of the ordinary control when the roll rate as sensed is equal to or greater than a predetermined threshold value. The second roll restraining control provides the damping-force characteristic of the shock absorber higher than that of the first roll restraining control.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to suspension systems for motorvehicles which ensure optimum control of the damping-forcecharacteristic of a shock absorber, and more particularly, to the art ofimproving riding comfort and stability of a vehicular position duringsteering operation.

2. Description of the Related Art

In connection with the known suspension systems for motor vehicles whichensure control of the damping-force characteristic of a shock absorber,JP-A 5-330325 shows a suspension control system. This system calculatesan absolute speed of vertical vibration of a vehicle body. In accordancewith the direction of the absolute speed, the system controls a dampingcoefficient of a shock absorber such that the contraction-side dampingcoefficient has a small value whereas the extension-side dampingcoefficient has a large value, or the contraction-side dampingcoefficient has a large value whereas the extension-side dampingcoefficient has a small value. By this, without detecting a relativespeed between a vehicle body located above the spring and an axlelocated below the spring, i.e. a sprung-unsprung relative speed,behavior of the vehicle body, i.e. sprung behavior, due to road inputcan be restrained.

With this known suspension control system, a damping performance of amotor vehicle can be secured with regard to sprung behavior due to roadinput. However, with regard to damping of behavior in the roll directionwhen a great lateral acceleration is applied to the vehicle body duringsteering operation, a controlling force is insufficient due to a momentof inertia in the roll direction being applied to the vehicle body.

Moreover, if the system is constructed to produce a damping forcesufficient to damp roll behavior during steering operation, an excessivedamping force is produced with respect to road input, resulting indeteriorated riding comfort during ordinary cruising.

It is, therefore, an object of the present invention to providesuspension systems for motor vehicles which ensure optimum control ofthe damping-force characteristic of a shock absorber in accordance withevery aspect of vehicular behavior occurring during non-steering andsteering operations to achieve both steering stability and ridingcomfort.

SUMMARY OF THE INVENTION

One aspect of the present invention lies in providing a method ofcontrolling a suspension system for a motor vehicle, said suspensionsystem including a shock absorber arranged between a vehicle body and awheel of the motor vehicle and having a device for allowing a change ina damping-force characteristic, the method comprising the steps of:

sensing a vertical behavior of the motor vehicle;

sensing a roll angle of the motor vehicle during steering operation;

sensing a roll rate of the vehicle during steering operation;

carrying out an ordinary control of the damping-force characteristic ofthe shock absorber in accordance with said vertical behavior as sensed;

carrying out a first roll restraining control in place of said ordinarycontrol when said roll angle as sensed is equal to or greater than apredetermined threshold value; and

carrying out a second roll restraining control in place of said ordinarycontrol when said roll rate as sensed is equal to or greater than apredetermined threshold value,

said second roll restraining control providing the damping-forcecharacteristic of the shock absorber higher than that of said first rollrestraining control.

Another aspect of the present invention lies in providing a suspensionsystem for a motor vehicle with a vehicle body and a wheel, comprising:

a shock absorber arranged between the vehicle body and the wheel andincluding a device for allowing a change in a damping-forcecharacteristic;

a first sensor for sensing a vertical behavior of the motor vehicle;

a second sensor for sensing a roll angle of the motor vehicle duringsteering operation;

a third sensor for sensing a roll rate of the motor vehicle duringsteering operation;

a control unit connected to said shock absorber and said first, second,and third sensors,

said control unit including a fundamental control part for carrying outan ordinary control of said damping-force characteristic of said shockabsorber in accordance with said vertical behavior as sensed,

said control unit including a first roll restraining control part forcarrying out a first roll restraining control in place of said ordinarycontrol when said roll angle as sensed is equal to or greater than apredetermined threshold value,

said control unit including a second roll restraining control part forcarrying out a second roll restraining control in place of said ordinarycontrol when said roll rate as sensed is equal to or greater than apredetermined threshold value,

said second roll restraining control providing said damping-forcecharacteristic of said shock absorber higher than that of said firstroll restraining control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a motor vehicle with a suspensionsystem embodying the present invention;

FIG. 2 is a block diagram showing the suspension system;

FIG. 3 is a longitudinal section showing a shock absorber;

FIG. 4 is an enlarged fragmentary section showing the shock absorber;

FIG. 5 is a graph showing the characteristic of damping force vs. pistonspeed;

FIG. 6 is a diagram showing the characteristic of damping force vs.pulse-motor stepping position;

FIGS. 7A-7C are sectional views taken along the line VII—VII in FIG. 4;

FIGS. 8A-8C are views similar to FIG. 7C, taken along the linesVIII—VIII and VIII′—VIII′ in FIG. 4;

FIGS. 9A-9C are views similar to FIG. 8C, taken along the line IX—IX inFIG. 4;

FIG. 10 is a view similar to FIG. 5, showing the damping-forcecharacteristic in the tension-side hard state;

FIG. 11 is a view similar to FIG. 10, showing the damping-forcecharacteristic in the tension-side and compression-side soft states;

FIG. 12 is a view similar to FIG. 11, showing the damping-forcecharacteristic in the compression-side hard state;

FIG. 13 is a view similar to FIG. 2, showing a signal processing circuitfor obtaining signals of sprung vertical speed and sprung-unsprungrelative speed;

FIG. 14A is a view similar to FIG. 12, showing a gain characteristic ofa signal of sprung vertical speed;

FIG. 14B is a view similar to FIG. 14A, showing a phase characteristicof a signal of sprung vertical speed;

FIG. 15 is a flowchart showing ordinary control for the damping-forcecharacteristic carried out by a control unit;

FIG. 16 is a time chart showing ordinary control for the damping-forcecharacteristic carried out by the control unit;

FIG. 17 is a map showing the characteristic of variable control gain vs.sprung-unsprung relative speed;

FIGS. 18A-18B are views similar to FIG. 15, showing switching betweenordinary control carried out by a fundamental control part andsteering-operation control carried out by a roll restraining controlpart;

FIG. 19 is a view similar to FIG. 16, showing switching between ordinarycontrol carried out by a fundamental control part and steering-operationcontrol carried out by a roll restraining control part; and

FIG. 20 is a table illustrating a control pattern of the shock absorber.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a suspension system for motor vehicles embodyingthe present invention comprises four shock absorbersSA_(FR),SA_(FL),SA_(RR),SA_(RR), each being arranged between a vehiclebody and a corresponding wheel. Note that the shock absorbers will bedesignated by SA simply when indicating the assemblage and explainingthe common structure. Also note that subscripts FR, FL, RR, RL designatefront-right, front-left, rear-right, and rear-left wheels, respectively.Moreover, the suspension system comprises a sprung vertical accelerationsensor (refer hereafter to as a vertical G sensor) 1(1_(FR),1_(FL),1_(RR),1_(RL)) arranged in each wheel position to sense asprung vertical acceleration G (G_(FR),G_(FL),G_(RR),G_(RL)) a steeringsensor 2 arranged with a steering ST to sense a steering angle Sθ, and acontrol unit 4 arranged in the vicinity of a driver's seat to output adrive control signal to a pulse motor 3 for each shock absorberSA_(FR),SA_(FL),SA_(RR),SA_(RR) in accordance with signals out of thevertical G sensor 1 and the steering sensor 2.

Referring to FIG. 2, the control unit 4 includes an interface circuit 4a, a central processing unit (CPU) 4 b, and a drive circuit 4 c. Theinterface circuit 4 a receives a signal of the sprung verticalacceleration G derived from the vertical G sensor 1 and a signal of thesteering angle Sθ derived from the steering sensor 2. In accordance withthose signals, the control unit 4 carries out control of thedamping-force characteristic of the shock absorber SA.

Referring to FIG. 3, the shock absorber SA includes a cylinder 30, apiston 31 for defining an upper and lower chambers A, B in the cylinder30, an outer tube 33 for defining a reservoir 32 at the outer peripheryof the cylinder 30, a base 34 for defining the lower chamber B and thereservoir 32, a guide member 35 for guiding slide movement of a pistonrod 7 connected to the piston 31, a suspension spring 36 interposedbetween the outer tube 33 and the vehicle body, and a bumper rubber 37.

Referring to FIG. 4, the piston 31 is formed with through holes 31 a, 31b. Compression-side and tension-side damping valves 20, 12 are arrangedto close the through holes 31 a, 31 b, respectively. A stud 38 arrangedthrough the piston 31 is engaged with and fixed to a bound stopper 41engaged with the head of the piston rod 7. The stud 38 is formed with acommunication hole 39 which forms a passage for communication betweenthe upper and lower chambers A, B in bypassing the through holes 31 a,31 b, i.e. a tension-side second passage E, a tension-side third passageF, a bypass passage G, and a compression-side second passage J as willbe described later. An adjuster 40 is rotatably arranged in thecommunication hole 39 to vary the sectional area of the passage.Tension-side and compression-side check valves 17, 22 are arranged atthe outer periphery of the stud 38 to allow and interrupt fluid flow onthe side of the passage formed by the communication hole 39 in thedirection of fluid flow. As shown in FIG. 3, the adjuster 40 is rotatedby the pulse motor 3 through a control rod 70. Moreover, the stud 38 isformed with first, second, third, fourth, and fifth ports 21, 13, 18,14, 16 in descending order.

The adjuster 40 is formed with a hollow 19, first and second transverseholes 24, 25 for communication between the inside and outside, and alongitudinal groove 23 formed on the outer periphery.

Four passages are formed between the upper and lower chambers A, B toallow fluid flow in the tension stroke: a tension-side first passage Dwhich extends to the lower chamber B via the through hole 31 b and thetension-side damping valve 12 with the inside opened; a tension-sidesecond passage E which extends to the lower chamber B via the secondport 13, the longitudinal groove 23, the fourth port 14, and thetension-side damping valve 12 with the outer periphery opened; atension-side third passage F which extends to the lower chamber B viathe second port 13, the longitudinal hole 23, the fifth port 16 and thetension-side check valve 17 opened; and a bypass passage G which extendsto the lower chamber B via the third port 18, the second transverse hole25, and the hollow 19. Three passages are formed between the upper andlower chambers A, B to allow fluid flow in the compression stroke: acompression-side first passage H which extends to the upper chamber Avia the through hole 31 a and the compression-side damping valve 20opened; a compression-side second passage J which extends to the upperchamber A via the hollow 19, the first transverse hole 24, the firstport 21, and the compression-side check valve 22 opened; and the bypasspassage G which extends to the upper chamber A via the hollow 19, thesecond transverse hole 25, and the third port 18.

Thus, the shock absorber SA is constructed so that rotation of theadjuster 40 can vary the damping-force characteristic in the multistageway with respect to both the tension and compression sides as shown inFIG. 5. Specifically, referring to FIG. 6, when rotating the adjuster 40counterclockwise with both the tension and compression sides set in thesoft state (refer hereafter to as a soft region SS), the tension-sidedamping-force characteristic can be varied in the multistage way,whereas the compression-side damping-force characteristic is fixed in alow damping-force region (refer hereafter to as a tension-side hardregion HS). On the other hand, when rotating the adjuster 40 clockwise,the compression-side damping-force characteristic can be varied in themultistage way, whereas the tension-side damping-force characteristic isfixed in a low damping-force region (refer hereafter to as acompression-side hard region SH).

FIGS. 7A-7C, 8A-8C, and 9A-9C show sectional views taken along the linesVII—VII, VIII—VIII and VIII′—VIII′, and IX—IX in FIG. 4, respectively,when the adjuster 40 is put in positions {circumflex over (1)},{circumflex over (2)}, {circumflex over (3)}. FIGS. 10-12 show thedamping-force characteristics in the positions {circumflex over (1)},{circumflex over (2)}, {circumflex over (3)}, respectively.

Referring to FIG. 13, a description will be made with regard to thestructure of a signal processing circuit of the control unit 4 forobtaining a sprung vertical speed Δx and a sprung-unsprung relativespeed (Δx−Δx₀).

At a block B1, using a formula of phase-lag compensation, the sprungvertical acceleration G (G_(FR),G_(FL),G_(RR),G_(RL)) sensed by thevertical G sensor 1 (1_(FR),1_(FL),1_(RR),1_(RL)) is converted into asignal of the sprung vertical speed Δx in a tower position.

A general formula of phase-lag compensation is given by atransfer-function formula (1):

G(s)=(AS+1)/(BS+1)  (1)

where A<B.

A formula of phase-lag compensation for obtaining the phase and gaincharacteristics equal to those obtained by an integral (1/S) in thefrequency band (0.5 to 3.0 Hz) necessary to control of the damping-forcecharacteristic and for reducing the gain in a low frequency (to 0.05 Hz)is given by a transfer-function formula (2):

G(s)=[(0.001S+1)/(10S+1)]×γ  (2)

where γ is a gain for adjusting the gain characteristic of a signalusing upon speed conversion by an integral (1/S). In this embodiment, γis set to 10. As a consequence, as seen from the gain characteristicgiven by a fully drawn line in FIG. 14A, and the phase characteristicgiven by a fully drawn line in FIG. 14B, only the gain in a lowfrequency is reduced without deteriorating the phase characteristic inthe frequency band (0.5 to 3.0 Hz) necessary to control of thedamping-force characteristic. Note that broken lines in FIGS. 14A-14Bshow the gain and phase characteristics of a signal of the sprungvertical speed Δx subjected to speed conversion by an integral (1/S).

At a block B2, a processing in a band-pass filter BPF is carried out toremove frequency components except those in a target frequency band tobe controlled. Specifically, the band-pass filter BPF comprises asecondary high-pass filter HPF (0.3 Hz) and a secondary low-pass filterLPF (4.0 Hz) to obtain a signal of the sprung vertical speed Δx(Δx_(FR),Δx_(FL),Δx_(RR),Δx_(RL)) directed to a sprung resonancefrequency band.

At a block B3, using a formula (3) of a transfer function Gu(s) from thesprung vertical acceleration G to the sprung-unsprung relative speed(Δx-Δx₀), a signal of the sprung-unsprung relative speed (Δx-Δx₀){(Δx-Δx₀)_(FR), (Δx-Δx₀)_(FL), (Δx-Δx₀)_(RR), (Δx-x₀)_(RL)} in a towerposition is obtained from a signal of the sprung vertical acceleration Gsensed by the vertical G sensor 1:

Gu(s)=−ms/(cs+k)  (3)

where m is a sprung mass, c is a damping coefficient of a suspension,and k is a spring constant of the suspension.

Referring to FIG. 15, a description will be made with regard to ordinarycontrol included in control of the damping-force characteristic in thecontrol unit 4 and carried out by a fundamental control part thereof.This ordinary control is carried out with respect to each shock absorberSA_(FR),SA_(FL),SA_(RR),SA_(RR).

At a step 101, it is determined whether or not the sprung vertical speedΔx has a positive value. If answer is YES, control proceeds to a step102 where the shock absorber SA is controlled in the tension-side hardregion HS. If answer is NO, control proceeds to a step 103.

At the step 103, it is determined whether or not the sprung verticalspeed Δx has a negative value. If answer is YES, control proceeds to astep 104 where the shock absorber SA is controlled in thecompressionside hard region SH. If answer is NO, control proceeds to astep 105.

At the step 105 which is taken when answer at the steps 101, 103 is NO,i.e. a value of the sprung vertical speed Δx is zero, the shock absorberSA is controlled in the soft region SS.

Referring to FIG. 16, control of the damping-force characteristic of theshock absorber SA will be described. When the sprung vertical speed Δxis varied as shown in FIG. 16, and when its value is zero, the shockabsorber SA is controlled in the soft region SS.

When a value of the sprung vertical speed Δx is positive, the shockabsorber SA is controlled in the tension-side hard region HS to fix thetension-side damping-force characteristic in the soft state. And thetension-side damping-force characteristic or a target damping-forcecharacteristic position Pt is changed in proportion to the sprungvertical speed Δx and in accordance with a formula (4):

Pt=α·Δx·Ku  (4)

where α is a tension-side constant, and Ku is a control gain having avalue which is variably set in inverse proportion to the sprung-unsprungrelative speed (Δx-Δx₀) and in accordance with a map showing thecharacteristic of the variable control gain vs. the sprung-unsprungrelative speed (Δx-Δx₀) as shown in FIG. 17.

When a value of the sprung vertical speed Δx is negative, the shockabsorber SA is controlled in the compression-side hard region SH to fixthe tension-side damping-force characteristic in the soft state. And thecompression-side damping-force characteristic or a target damping-forcecharacteristic position Pc is changed in proportion to the sprungvertical speed Δx and in accordance with a formula (5):

Pc=β·Δx·Ku  (5)

where β is a compression-side constant.

Referring to FIG. 16, a description will be made with regard toswitching of the control region of the shock absorber SA, which isincluded in control of the damping-force characteristic in the controlunit 4.

Referring to FIG. 16, in a region a, the sprung vertical speed Δx isswitched from a negative value (downward direction) to a positive value(upward direction). In the region a, the sprung-unsprung relative speed(Δx-Δx₀) has a negative value (the shock absorber SA is in thecompression stroke), so that the shock absorber SA is controlled in thetension-side hard region HS in accordance with the direction of thesprung vertical speed Δx. Thus, in the region a, the compression strokein which the shock absorber SA is at that time shows the softcharacteristic.

In a region b, the sprung vertical speed Δx has a positive value (upwarddirection), and the sprung-unsprung relative speed (Δx-Δx₀) is switchedfrom a negative value to a positive value (the shock absorber SA is inthe tension stroke), so that the shock absorber SA is controlled in thetension-side hard region HS in accordance with the direction of thesprung vertical speed Δx. And the shock absorber SA is in the tensionstroke. Thus, in the region b, the tension stroke in which the shockabsorber SA is at that time shows the hard characteristic proportionalto a value of the sprung vertical speed Δx.

In a region c, the sprung vertical speed Δx is switched from a positivevalue (upward direction) to a negative value (downward direction).However, the sprung-unsprung relative speed (Δx-Δx₀) has a positivevalue (the shock absorber SA is in the tension stroke), so that theshock absorber SA is controlled in the compression-side hard region SHin accordance with the direction of the sprung vertical speed Δx. Thus,in the region c, the tension stroke in which the shock absorber SA is atthat time shows the soft characteristic.

In a region d, the sprung vertical speed Δx has a negative value(downward direction), and the sprung-unsprung relative speed (Δx-Δx₀) isswitched from a positive value to a negative value (the shock absorberSA is in the tension stroke), so that the shock absorber SA iscontrolled in the compression-side hard region SH in accordance with thedirection of the sprung vertical speed Δx. And the shock absorber SA isin the compression stroke. Thus, in the region d, the compression strokein which the shock absorber SA is at that time shows the hardcharacteristic proportional to a value of the sprung vertical speed Δx.

As described above, in this embodiment, when the sprung vertical speedΔx and the sprung-unsprung relative speed (Δx-Δ₀) have the same sign,i.e. in the regions d, d, the stroke in which the shock absorber SA isat that time is controlled to show the hard characteristic. When theyhave different signs, i.e. in the regions a, c, the stroke in which theshock absorber SA is at that time is controlled to show the softcharacteristic. That is, the same control as control of thedamping-force characteristic based on the skyhook theory is carried out.Moreover, in this embodiment, when switching the stroke of the shockabsorber SA, i.e. when passing from the region a to the region b, andfrom the region c to the region d, i.e. from the soft characteristic tothe hard characteristic, the damping-force characteristic position ofthe coming stroke is already switched to the hard characteristic in theprevious region a, c, obtaining switching from the soft characteristicto the hard characteristic without time lag.

The control unit 4 includes a signal processing circuit for obtaining aroll angle Rθ and a roll rate RV. Specifically, in this embodiment, theroll angle Rθ is obtained by filtering a signal of the steering angle Sθsensed by the steering sensor 2. The roll rate RV is obtained bydifferentiating a signal of the steering angle Sθ. Values of the rollangle Rθ and the roll rate RV are corrected in accordance with a vehiclespeed.

Referring to FIGS. 18A-18B, a description will be made with regard toswitching between ordinary control carried out by the fundamentalcontrol part and steering-operation control carried out by a rollrestraining control part.

Referring to FIG. 18A, at a step 201, it is determined whether or not anabsolute value |RV| of a roll-rate signal is equal to or greater than aroll-rate on threshold value. If answer is YES, control proceeds to astep 202 where a roll-rate control flag is set, and count of a roll-rateoff timer or second timer is cleared to zero, then, control proceeds toa step 203. At the step 201, if answer is NO, control proceeds to a step203.

At the step 203, it is determined whether or not an absolute value |Rθ|of a roll-angle signal is equal to or greater than a roll-angle onthreshold value. If answer is YES, control proceeds to a step 204 wherea roll-angle control flag is set, and count of a roll-angle off timer orfirst timer is cleared to zero, then, control proceeds to a step 205. Atthe step 203, if answer is NO, control proceeds to a step 205.

At the step 205, it is determined whether or not the roll-rate controlflag is set. If answer is YES, control proceeds to a step 206. At thestep 206, it is determined whether or not the absolute value |RV| of aroll-rate signal is smaller than a roll-rate off threshold value. Ifanswer is YES, control proceeds to a step 207 where count of theroll-rate off timer or second timer is incremented by one, then, controlproceeds to a step 208.

At the step 208, it is determined whether or not count of the roll-rateoff timer or second timer is greater than a predetermined value or timer2. If answer is YES, control proceeds to a step 209 where the roll-ratecontrol flag is cleared. At a subsequent step 210, count of theroll-rate off timer is cleared to zero, then, control proceeds to a step211 in FIG. 18B.

At the step 206, if answer in NO, control proceeds to the step 210. Andat the step 205 or 208, if answer is NO, control proceeds to the step211.

Referring to FIG. 18B, at the step 211, it is determined whether or notthe roll-angle control flag is set. If answer is YES, control proceedsto a step 212. At the step 212, it is determined whether or not theabsolute value |Rθ| of a roll-angle signal is smaller than a roll-angleoff threshold value. If answer is YES, control proceeds to a step 213where count of the roll-angle off timer or first timer is incremented byone, then, control proceeds to a step 214.

At the step 214, it is determined whether or not count of the roll-angleoff timer is greater than a predetermined value or timer 1. If answer isYES, control proceeds to a step 215 where the roll-angle control flag iscleared. At a subsequent step 216, count of the roll-angle off timer iscleared to zero, then, control proceeds to a step 217.

At the step 212, if answer is NO, control proceeds to the step 216. Andat the step 211 or 214, if answer is NO, control proceeds to a step 217.

At the step 217, it is determined whether or not the roll-rate controlflag is set. If answer is YES, control proceeds to a step 218 whereswitching to roll-rate restraining control is carried out, then, oneflow is completed. If answer is NO, control proceeds to a step 219.

At the step 219, it is determined whether or not the roll-angle controlflag is set. If answer is YES, control proceeds to a step 220 whereswitching to roll-angle restraining control is carried out. If answer isNO, control proceeds to a step 221 where switching to ordinary controlis carried out, then, one flow is completed.

The above control flow is repeatedly carried out thereafter.

Referring to FIG. 19, a further description will be made with regard toswitching between ordinary control carried out by the fundamentalcontrol part and steering-operation control carried out by the rollrestraining control part.

A) During normal cruising

When steering operation is not carried out, or roll behavior produced bysteering operation is smaller than a predetermined value, ordinarycontrol is carried out by the fundamental control part, achievingcontrol of the damping-force characteristic with riding comfortrespected.

B) During steering operation

When roll behavior produced by steering operation is greater than thepredetermined value, steering-operation control is carried out, in placeof ordinary control, by a roll-angle restraining control part or aroll-rate restraining control part.

Specifically, when a value of the roll angle Rθ is equal to or greaterthan the roll-angle on threshold value, switching is carried out fromordinary control to roll-angle restraining control. As soon as apredetermined timer period elapses after a value of the roll angle Rθ issmaller than the roll-angle off threshold value, switching to ordinarycontrol is carried out.

When the roll rate RV is equal to or greater than the roll-rate onthreshold value, switching is carried out from ordinary control androll-angle restraining control to roll-rate restraining control. As soonas a predetermined timer period elapses after a value of the roll rateRV is smaller than the roll-rate off threshold value, switching toordinary control or roll-angle restraining control is carried out.

During roll-angle restraining control, the damping-force characteristicof the shock absorber SA shows higher value than during ordinarycontrol, restraining vehicular roll during steady turn withoutdeteriorating riding comfort so much, securing steering stability.

During roll-rate restraining control, the damping-force characteristicof the shock absorber SA shows higher value than during roll-anglerestraining control, restraining transient vehicular roll during quicksteering operation, securing steering stability.

Two examples of steering-operation control in the steering-operationcontrol part will be described.

In the example 1, the damping-force characteristic positions Pt, Pc ofthe shock absorber SA are fixed to any hard-characteristic positionspreviously set. The fixed positions are controlled independently withrespect to each wheel during both roll-angle restraining control androll-rate restraining control.

In the example 2, the damping-force characteristic positions Pt, Pc isvariably set in real time in accordance with a value of the sprungvertical speed Δx so that the stroke of the shock absorber SA shows apredetermined hard characteristic.

Maximum and minimum values of the damping-force characteristic positionsare controlled independently with respect to each group of front andrear wheels during both roll-angle restraining control and rollraterestraining control.

FIG. 20 shows four combined examples of the examples 1 and 2 duringroll-angle restraining control and roll-rate restraining control.

Having described the present invention in connection with the preferredembodiment, it is noted that the present invention is not limitedthereto, and various changes and modifications can be made withoutdeparting from the scope of the present invention.

By way of example, in the embodiment, the roll angle and the roll rateare obtained from a steering-angle signal, alternatively, they can beobtained from other signal such as a wheel-speed signal, alateral-acceleration signal, or a yaw-rate signal.

Moreover, in the embodiment, the shock absorber is controlled in thesoft region SS only when a signal of sprung vertical speed is zero.Alternatively, a predetermined insensitive band may be arranged aroundzero to maintain the damping-force characteristic in the soft region SSas far as a sprung vertical speed is varied within the band. This canprevent control hunting.

What is claimed is:
 1. A method of controlling a suspension system for amotor vehicle, said suspension system including a shock absorberarranged between a vehicle body and a wheel of the motor vehicle andhaving a device for allowing a change in a damping-force characteristic,the method comprising the steps of: sensing a vertical behavior of themotor vehicle; sensing a roll angle of the motor vehicle during steeringoperation; sensing a roll rate of the vehicle during steering operation;carrying out a first control of the damping-force characteristic of theshock absorber in accordance with said vertical behavior as sensed;carrying out a first roll restraining control in place of said firstcontrol when said roll angle as sensed is equal to or greater than apredetermined threshold value; and carrying out a second rollrestraining control in place of said first control when said roll rateas sensed is equal to or greater than a predetermined threshold value,said second roll restraining control providing the damping-forcecharacteristic of the shock absorber higher than that of said first rollrestraining control.
 2. A method as claimed in claim 1, wherein saidsecond roll restraining control step is carried out prior to said firstroll restraining control step.
 3. A method as claimed in claim 1,wherein said predetermined threshold value of said roll angle includesan ON threshold value for restarting said first roll restraining controland an OFF threshold value for interrupting said first roll restrainingcontrol, and said predetermined threshold value of said roll rateincludes an ON threshold value for starting said second roll restrainingcontrol and an OFF threshold value for interrupting said second rollrestraining control.
 4. A method of controlling a suspension system fora motor vehicle, said suspension system including a shock absorberarranged between a vehicle body and a wheel of the motor vehicle andhaving a device for allowing a change in a damping-force characteristic,the method comprising the steps of: sensing a vertical behavior of themotor vehicle; sensing a roll angle of the motor vehicle during steeringoperation; sensing a roll rate of the vehicle during steering operation;carrying out a first control of the damping-force characteristic of theshock absorber in accordance with said vertical behavior as sensed;carrying out a first roll restraining control in place of said firstcontrol when said roll angle as sensed is equal to or greater than apredetermined threshold value; and carrying out a second rollrestraining control in place of said first control when said roll rateas sensed is equal to or greater than a predetermined threshold value,said second roll restraining control providing the damping-forcecharacteristic of the shock absorber higher than that of said first rollrestraining control; wherein said predetermined threshold value of saidroll angle includes an ON threshold value for restarting said first rollrestraining control and an OFF threshold value for interrupting saidfirst roll restraining control, and said predetermined threshold valueof said roll rate includes an ON threshold value for starting saidsecond roll restraining control and an OFF threshold value forinterrupting said second roll restraining control; and furthercomprising the steps of: cancelling said first roll restraining controlas soon as a predetermined period of time elapses after said roll angleas sensed is smaller than said OFF threshold value of said roll angle;and cancelling said second roll restraining control as soon as apredetermined period of time elapses after said roll rate as sensed issmaller than said OFF threshold value of said roll rate.
 5. A method asclaimed in claim 4, wherein a correction of said on threshold values andoff threshold values of said roll angle and said roll rate is carriedout in accordance with a vehicle speed.
 6. A method as claimed in claim5, wherein a content of said correction of said on threshold values andoff threshold values with respect to a vehicle speed is switched by aselector switch.
 7. A suspension system for a motor vehicle with avehicle body and a wheel, comprising: a shock absorber arranged betweenthe vehicle body and the wheel and including a device for allowing achange in a damping-force characteristic; a first sensor for sensing avertical behavior of the motor vehicle; a second sensor for sensing aroll angle of the motor vehicle during steering operation; a thirdsensor for sensing a roll rate of the motor vehicle during steeringoperation; a control unit connected to said shock absorber and saidfirst, second, and third sensors, said control unit including afundamental control part for carrying out a first control of saiddamping-force characteristic of said shock absorber in accordance withsaid vertical behavior as sensed, said control unit including a firstroll restraining control part for carrying out a first roll restrainingcontrol in place of said first control when said roll angle as sensed isequal to or greater than a predetermined threshold value, said controlunit including a second roll restraining control part for carrying out asecond roll restraining control in place of said first control when saidroll rate as sensed is equal to or greater than a predeterminedthreshold value, said second roll restraining control providing saiddamping-force characteristic of said shock absorber higher than that ofsaid first roll restraining control.
 8. A suspension system as claimedin claim 7, wherein said second roll restraining control is carried outprior to said first roll restraining control.
 9. A suspension system asclaimed in claim 7, wherein said predetermined threshold value of saidroll angle includes an ON threshold value for starting said first rollrestraining control and an OFF threshold value for interrupting saidfirst roll restraining control, and said predetermined threshold valueof said roll rate includes an ON threshold value for starting saidsecond roll restraining control and an OFF threshold value forinterrupting said second roll restraining control.
 10. A suspensionsystem for a motor vehicle with a vehicle body and a wheel, comprising:a shock absorber arranged between the vehicle body and the wheel andincluding a device for allowing a change in a damping-forcecharacteristic; a first sensor for sensing a vertical behavior of themotor vehicle; a second sensor for sensing a roll angle of the motorvehicle during steering operation; a third sensor for sensing a rollrate of the motor vehicle during steering operation; a control unitconnected to said shock absorber and said first, second, and thirdsensors, said control unit including a fundamental control part forcarrying out a first control of said damping-force characteristic ofsaid shock absorber in accordance with said vertical behavior as sensed,said control unit including a first roll restraining control part forcarrying out a first roll restraining control in place of said firstcontrol when said roll angle as sensed is equal to or greater than apredetermined threshold value, said control unit including a second rollrestraining control part for carrying out a second roll restrainingcontrol in place of said first control when said roll rate as sensed isequal to or greater than a predetermined threshold value, said secondroll restraining control providing said damping-force characteristic ofsaid shock absorber higher than that of said first roll restrainingcontrol, and wherein said predetermined threshold value of said rollangle includes an ON threshold value for starting said first rollrestraining control and an OFF threshold value for interrupting saidfirst roll restraining control, and said predetermined threshold valueof said roll rate includes an ON threshold value for starting saidsecond roll restraining control and an OFF threshold value forinterrupting said second roll restraining control further comprising: afirst timer for cancelling said first roll restraining control as soonas a predetermined period of time elapses after said roll angle assensed is smaller than said OFF threshold value of said roll angle; anda second timer for cancelling said second roll restraining control assoon as a predetermined period of time elapses after said roll rate assensed is smaller than said OFF threshold value of said roll rate.
 11. Asuspension system as claimed in claim 10, wherein a correction of saidon threshold values and off threshold values of said roll angle and saidroll rate is carried out in accordance with a vehicle speed.
 12. Asuspension system as claimed in claim 11, wherein a content of saidcorrection of said on threshold values and off threshold values withrespect to a vehicle speed is switched by a selector switch.